Micromachining using high energy light ions

ABSTRACT

Structures of microminiature dimensions are formed by scanning a nearly parallel beam of high energy light ions across the surface of a resist material such as PMMA in a predetermined pattern. The resulting chemical changes in the exposed resist material allows a chemical developer to remove the exposed material while leaving the unexposed material substantially unaffected. In addition because the ions have a well defined range in the material depending on their energy, the resist can be exposed to a predetermined well defined depth. By this method, resist structures of three dimensional complexity can be micromachined. This is achieved by simultaneously scanning the beam and orienting the resist layer in a controlled manner. Further enhancement may be achieved by the use of multiple deposition and exposure of resist layers. These resist microstructures may be further utilized to produce microstructures in other materials by the application of processes such as electroplating and micromoulding.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/SG97/00057 which has an Internationalfiling date of Nov. 6, 1997 which designated the United States ofAmerica.

1. Field of the Invention

This invention relates to the area of manufacturing components anddevices for micro-mechanical, micro-optical, micro-fluidic,micro-electronic, micro-acoustical, and micro-chemical applications. andutilises high energy light ions for micromachining. It can be appliedeither independently of, or in combination with, other techniques formicromachining.

2. Background of the Invention

The prior art which relates to the present invention is the use ofX-rays for micromachining, commonly referred to by its German acronymLIGA(E. W. Becker, et.al., Microelectronic Engineering 4(1986)35-56; W.Ehrfeld and H. Lehr, Radiat. Phys. Chem. 45(1995)349-365), which in itsmost common embodiment comprises four main process steps:

i) In the first step a layer of positive polymer resist, usuallypolymethylmethacrylate (PMMA), which is typically several hundredmicrons thick and adhering to a metal substrate, is exposed in a deepX-ray lithography process using X-rays (usually from a synchrotronsource) through a proximity X-ray mask. The use of a planar mask withdefined areas of high and low transparency to X-rays is essential tothis step.

ii) In the second step a suitable chemical developer is used to removethe exposed volume of the resist and expose selected areas of theunderlying metal substrate. This chemical developer must be highlyspecific in completely removing the well exposed regions of resist whileleaving unexposed or marginally exposed resist unaffected (V. Ghica andW. Glashauser, Verfahren fuer die spannungsfreie Entwicklung vonbestrahlten Polymethylmethacrylat-Schichten, Offenlegungsschrift DE3039110 Siemens AG, Munich).

iii) The third step is to electroplate metal onto the exposed metalsubstrate until the deposited metal thickness is equal to the resistthickness. The remaining resist is then removed to leave metalstructures protruding from the metal substrate.

iv) In a fourth step these metal structures may be used as a mould toform structures in other materials.

The merits of the LIGA process lie chiefly in the ability of the firsttwo process steps to form microstructures of large structural height(tens of microns to a few mm), with aspect-ratios up to 100. The aspectratio is defined as the ratio of the structural height to the smallestlateral dimension. In typical practice, polymer structures with lateraldimensions of several microns to several hundred microns are formed byprocess steps one and two as described above. The limitations of theLIGA process which are relevant to the present invention are associatedwith the first process step of deep X-ray lithography using a proximityX-ray mask, and consist of the following:

a) That without undue complexity such a process is only suitable for theproduction of prismatic polymer structures on a planar base with wallsperpendicular to the planar base.

b) That in applications where it is desirable that the deposition ofenergy by the exposure process should be of limited range or depth thatthe X-ray exposure process is unsuitable. This is due to the fact thatX-rays are attenuated by matter, but do not have a fixed or well definedrange.

c) That the cost and effort involved in fabricating a mask for the X-rayexposure step is high for the fabrication of prototype and low volume ofproduction microstructures.

d) That the adhesion of the resist structures to the substrate can beadversely effected by the undesirable exposure of the resist due tophotoelectrons, auger electrons, and fluorescence x-rays emitted fromthe substrate following absorption of x-rays from the primary source (F.J. Pantenburg, et.al., Microelectronic Engineering 23(1994)223-226; F.J. Pantenburg and J. Mohr, Nucl. Instrum. and Meth. B97(1995)551-556)

OBJECTIVE OF THE INVENTION

It is a primary object of this invention to provide a means of exposinga resist, for the purpose of micromachining, by using a direct-writebeam of energetic ions. Specifically, the type of ions employed in thisinvention are the isotopes of hydrogen, helium or lithium, with kineticenergies in excess of about 250 KeV.

Another object of this invention is to provide a means of creatingmicrostructures in a resist many microns thick (eg greater than about 2microns), which can either be of practical use in themselves, or to formmicrostructures in other materials, for example by electroplating onto ametal substrate.

It is a further object of the present invention that the microstructurescreated in a resist several microns thick (eg in the range 2 to 20microns) can be of high-aspect-ratio (i.e. the height of themicrostructures is large in comparison to their lateral dimensions).

It is another object of this invention to provide a means of exposingresist which overcomes many of the limitations associated with priorart, namely the deep X-ray lithography process which is the first stepin the LIGA process.

Specific advantages of the present invention over prior art include, butare not limited to, a) the greater geometrical freedom in themicrostructures which can be machined, b) reduction or elimination ofdamage to material underlying the resist, c) the ability to machinestructures with sub-micron dimensions in resists of many micronsthickness and d) the ability to micromachine structures withoutrequiring a mask. Other objects, features and advantages of the presentinvention will become apparent from the detailed description whichfollows, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows the effect of exposing the resist 1 to high energy lightions 2 of different energy levels E₁ and E₂ (E₁>E₂) followed bydevelopment of the exposed resist. The exposure is achieved using a beamof high energy light ions which are electrically or magnetically focusedto produce a nearly parallel beam. Selective areas 3 of the resist 1 areexposed through the relative scanning motion between the beam and theresist 1.

FIG. 1b shows the resist 1 after development of the exposed resist.

FIG. 2 shows structures in a resist 1 containing blind features andfeatures formed by a sequence of exposure and development steps.

FIG. 2a: shows features formed by first exposure and development ofresist 1.

FIG. 2b: shows a second exposure of the resist 1, scanning over thedeveloped resist with the high energy light ions 2 to selectively exposeareas 3.

FIG. 2c: shows the features following second development of resist 1.

FIG. 3 shows the formation of metal microstructures of high aspect ratioonto a metal layer 4 over a substrate 5.

FIG. 3a: shows the metallised substrate 4 & 5 with layer of resist 1 ofuniform thickness which has already been selectively exposed 3.

FIG. 3b: shows the electroplated metal 6 formed in the structure afterdevelopment and electroplating.

FIG. 3c: shows the metal microstructures 7 formed on the metallisedsubstrate 4 & 5 after grinding or polishing and removal of the remainingresist. The metal microstructures 7 so formed may have a high aspectratio as shown.

FIG. 4 illustrates the use of multiple steps of the basic process toproduce structures of greater geometrical complexity.

FIG. 4a: shows the exposed areas 3 of the resist 1.

FIG. 4b: shows above after deposition of another layer of resist 1.

FIG. 4c: shows a second exposure.

FIG. 4d: shows the features formed after the terminating development.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

Broadly according to this invention there is provided a method forexposing a defined area of a resist material to produce a change in aproperty of said area enabling said area to be selectively acted on,characterised in that the exposure is effected using a high energy beamof light ions.

In the present invention, the exposure of the resist is achieved using abeam of high energy (e.g. between about 250 KeV to about 25 MeV) lightions (e.g. hydrogen, deuterium, tritium, helium mass three, helium massfour, lithium, beryllium., boron and carbon) (see FIG. 1a). In generalthe ions in the beam will be of a specific type and of a well definedenergy, and the resist will be a suitable positive polymer resist suchas PMMA. In many embodiments of the invention the ions will beelectrically or magnetically focused to produce a nearly parallel beam,with a well defined range of diameters typically within the range of 100nanometers to 10 micrometers.

In order to expose regions of the resist in a specified pattern (whichcan subsequently be developed chemically to produce resistmicrostructures) a means of scanning the beam over the resist surface isemployed. Hence the present invention is a direct-write process, and nomask is required. This scanning system can be a magnetic orelectrostatic beam deflection system whose operation is under computercontrol, or the computer controlled movement of the resist surfacerelative to the ion beam. The resist is developed using a highlyspecific chemical developer which completely removes the exposed regionsof resist while leaving unexposed or marginally exposed resistsubstantially unaffected (V. Ghica and W. Glashauser) (see FIG. 1b).Moreover following a first exposure and development of the resist, thereis no impediment to repeating the exposure and development of the sameresist a multiplicity of times.

The present invention can be used to machine resist microstructures of asimple prismatic geometry as with prior art. For resists of severalmicrons thickness the present invention enables microstructures withsub-micron lateral dimensions to be machined. Depending on the thicknessof the resist the achievable aspect-ratio can be greater than thatattainable by any other prior art process. This aspect of the presentinvention will be of considerable value in the fabrication of X-raymasks for deep X-ray lithography (as used in the LIGA process), asconsiderable difficulties are experienced using prior art techniques infabricating masks of a sufficient thickness (F. J. Pantenburg, et.al; F.J. Pantenburg and J. Mohr).

One of the principal advantages of the present invention over prior artis that it also enables the machining of resist microstructures having amore complex three dimensional geometry. This can be achieved by severalmeans as will described in greater detail below, but in each case thisincreased geometrical freedom arises from the action of two of thedistinctive features of the present invention.

i) The first of these distinctive features is that the present inventionis a direct-write process using a scanned beam. Hence not only can thebeam be moved in a predetermined pattern over the surface of the resist,but it can also be orientated at any angle of incidence to the resistsurface. This orientation of the beam is more conveniently achieved byangular movements of the resist material (along with any underlyingmaterial or object) rather than by changing the direction of the beamincident on a stationary resist.

ii) The second of the distinctive features of this invention is thatenergetic ions of a given type and of a single well defined kineticenergy have a finite and well defined mean range in the resist material.The ranges of individual ions exhibit only a comparatively smallstatistical variation about this mean range (typically a few percent).For example, for PMMA resist of density 1.2 g/cm3 protons with kineticenergy of 2.0 MeV have a mean range of about 62 microns, and 3.0 MeVprotons a mean range of about 122 microns. For the purposes of thepresent invention this means that the exposure of the resist materialends at a range slightly beyond the mean range of the selected incidentions, and therefore blind holes, slots and other geometrical featurescan be formed in the resist material. It is of significance to thepresent invention that the range of the ions in the beam can be modifiedappropriately by a suitable choice of ion type and kinetic energythereof, thereby determining the depth to which the resist is exposed(see FIG. 1a, 1 b). For light ions with kinetic energies in excess ofabout 250 KeV, the paths of the ions in the resist material are ofsufficient range and the deviations from a straight line path aresufficiently low as to make them of use for the purposes ofmicromachining as in the present invention.

In some embodiments of the invention a sequence of exposure anddevelopment steps will be employed to create voids in the resist with amore complex three dimensional shape than can be created in a singleexposure and development process. An embodiment of the invention whichcontains blind features and features formed by a sequence of exposureand development steps is illustrated in FIG. 2.

In some embodiments of the present invention. a metal surface will liebelow the resist layer, and the invention will be put into effect byexposing certain areas of the resist layer to the full depth of theresist, i.e. the ions in the beam penetrate to the metal layer.Following development in a suitable chemical developer, metal iselectroplated onto the exposed areas of the metal surface to the samedepth as the resist layer. A grinding/polishing process may be used toremove metal deposited above the resist surface. The remaining resist isthen removed to leave metal microstructures protruding from the metalsubstrate. In the present invention, metal microstructures can also beformed on a variety of non-metallic substrates, including silicon,germanium, other semiconducting materials, ceramic, glass and so forth,by first applying a metallic layer to the substrate (by for examplesputtering) followed by a layer of resist to be structured. In general aplanar substrate will be used and the resist layer will be of uniformthickness. Following structuring of the resist, metal structures wouldbe formed in the manner described above. This process is illustrated inFIG. 3. For applications involving a metallic or metallised substrate, aparticular advantage of the present invention over prior art is that thesecondary radiations (secondary electrons, photoelectrons, augerelectrons, and fluorescence x-rays) in the vicinity of the metal-resistinterface is substantially lower than would be the case when X-rays areused as the primary means of exposure (ie. as for deep X-raylithography, as used in LIGA), therefore greatly reducing problemsassociated with resist adhesion (F. J. Pantenburg, et.al; F. J.Pantenburg and J. Mohr). In some embodiments of the invention there maybe no substrate material at all; the whole device or component to bemachined being composed entirely of the resist material.

One particular embodiment of the invention would be the use of asemiconducting substrate on which electronic devices had previously beenfabricated. Metal structures would then be formed on this substrate asdescribed above, which would be in electrical contact with theunderlying electronic devices. In this manner, the present inventioncould be used to fabricate sensor and actuator devices with integratedelectronics. A particular advantage of the present invention forfabricating such integrated devices is that the range of the ions can besuitably chosen such that radiation damage or unwanted implantation ofthe underlying semiconducting material is avoided.

In some embodiments of the present invention the developed resiststructures will have walls only perpendicular to the original resistsurface. If the resist layer is of uniform thickness and adhering to ametal substrate this will permit the electroplating of metal structureswith walls perpendicular to the metal substrate. While in otherembodiments of the present invention the walls may be at various anglesto the original resist surface, this having been achieved by arrangingfor the ion beam to be incident at various angles to the original resistsurface, and at varying positions on the surface of the resist. In afurther embodiment of the invention curved resist microstructure wallsmay be formed by coordinated movements and orientation of the ion beam.In embodiments of the present invention which employ a metal substratethis would enable metal structures with a complex three dimensionalgeometry to be created by electroplating.

In other embodiments of the present invention resist structures ofgreater geometrical complexity can be made with the use of a multi-stepprocesses, in which a succession of resist layer deposition and exposuresteps are terminated with a single development of all exposed volumes ofresist. Such a multi-step process could comprise: repeated exposuresteps (without intervening development); additions of further resistlayers to the existing resist surface; variation of the scanned patternand orientation of the ion beam for each resist layer; and variation ofthe energy or type of ions employed for the exposure step in order toselect depth of penetration. At the end of such a multi-step process afinal development step would be used to remove all exposed volumes ofthe resist which are accessible to the chemical developer. FIG. 4illustrates such an embodiment of the present invention. In embodimentsof the invention which employ a metallic or metallised substrate, theapplication of the above described multi-step process would enable metalstructures of almost arbitrary geometrical complexity to beelectroplated onto the surface of the substrate.

In all embodiments of the present invention in which metal structuresare formed by means of electroplating, these metal microstructure couldbe further utilised to produce microstructures in other materials bymicromoulding, as is sometimes done in prior art techniques. Otherembodiments of the present invention could use one or more sacrificiallayers in combination with electroplating to produce metal structureswhich are partially or wholly detached from the underlying metalsubstrate.

In all embodiments of the present invention described above it isassumed that a positive resist material would be employed (such as PMMA)for which the exposed regions of the resist material are removed by achemical developer. But the present invention could equally be put intoeffect with the use of a suitable negative resist material (anddeveloper) for which the unexposed regions of the developer are removedby the chemical developer and the exposed regions remain substantiallyintact.

In many of the above described embodiments, the invention is performedusing a resist layer of uniform thickness, which adheres to anunderlying planar substrate of some other material, but the practice ofthe invention does not preclude the use of objects which have a morecomplicated three dimensional shape, whither these objects be composedsolely of resist material or comprise both resist and materials of othertypes. In particular some embodiments of the invention ion beammicromachining could be performed on a resist material which had alreadybeen structured using some other method of micromachining, for exampledeep X-ray lithography. Such an embodiment of the invention wouldexploit the advantageous features of the prior micromachining process,for example the large structural height which can be achieved usingLIGA, and extend its range of application by exploiting the strengths ofthe present invention, namely the greater geometrical freedom and finiterange of exposure.

Generally all embodiments of the invention will employ a magnetic orelectrostatic beam deflection system under computer control, to scan theion beam over the resist material such as to write (expose) apredetermined pattern in the resist. Generally the facility to blank thebeam (i.e. rapidly switch the beam intensity to zero) is required, andmust also be computer controlled, so that separated enclosed regions ofexposure can be written. In some embodiments of the invention theexposure may be made with a single scan of the predetermined pattern,while in other embodiments of the invention the scan pattern may bere-written a multiplicity of times in precisely the same region ofresist. Re-writing the pattern a multiplicity of times has the advantageof averaging out variations in beam intensity which otherwise wouldresult in poor uniformity in the desired exposure. Generally thearea-of-coverage of a magnetic or electrostatic beam deflection systemis quite limited. By area-of-coverage it is meant the actual surfacearea of resist at normal incidence to the beam, over which the ion beamcan be scanned without significantly degrading the fine focusing of thebeam. The area-of-coverage may in a some embodiments of the invention bea square with a side of approximately 1 mm. In some embodiments of theinvention a greater coverage than can be conveniently attained with abeam deflection system may be required. These embodiments of theinvention will employ not only a beam deflection system as a primarymeans of scanning a pattern, but additionally a two or three axistranslation stage to move the object to which the resist is attached.The movements of the stage will be computer controlled and coordinatedwith the operation of the beam deflection and blanking system. Inembodiments of the invention in which microstructure walls at varyingangles to the resist surface are to be machined the translationalmotions of a three axis stage are combined with a mechanism fororientating the object to which the resist is attached with threeangular degrees of motion. The actions of this six axis system will becomputer controlled and coordinated with the operation of the beamdeflection and blanking system.

What is claimed is:
 1. A method for preparing a component comprisingexposing a defined area of a maskless resist material to produce achemical change in a property of said area enabling said area to beselectively removed or rendered inert wherein the defined area isexposed with a high energy beam of light ions, the beam having an energygreater than 250 KeV.
 2. A method in accordance with claim 1 whereinsaid change is acted on by a chemical developer.
 3. A method inaccordance with claim 2, wherein said chemical developer selectivelyremoves the exposed resist material.
 4. A method in accordance withclaim 1, wherein the said exposure produces a change in a property ofsaid defined area exposed to the high energy beam of light ionsrendering said defined area inert, and wherein an area not exposed tothe high energy ions is selectively acted on.
 5. A method in accordancewith claim 1, wherein the resist material comprises a positive polymerresist.
 6. A method in accordance with claim 5, wherein the positiveresist is polymethylmethacrylate (PMMA).
 7. A method in accordance withclaim 1, wherein the ions are electrically or magnetically focused toproduce a parallel beam, which is scanned over the surface of the resistmaterial in a predefined pattern thereby producing three dimensionalmicrostructures in the developed material resist.
 8. A method inaccordance with claim 7, wherein the microstructures produced in thedeveloped resist are used for the purposes of fabricating a mask forX-ray lithography as used in a LIGA or other lithographic process.
 9. Amethod in accordance with claim 8, wherein the parallel beam has adiameter within the range of about 100 nanometers to 10 micrometers. 10.A method in accordance with claim 7, wherein the beam is at an angle tothe resist surface which is changed in a controlled fashion during thescanning of the beam.
 11. A method in accordance with claim 10, whereinthe parallel beam has a diameter within the range of about 100nanometers to 10 micrometers.
 12. A method in accordance with claim 7,wherein microstructures are produced in the exposed resist, having adepth which is determined by the selection of ion type, energy and angleof incidence on the resist surface.
 13. A method in accordance withclaim 12, wherein the microstructures are holes, slots or voids of othergeometries.
 14. A method in accordance with claim 12, wherein theparallel beam has a diameter within the range of about 100 nanometers to10 micrometers.
 15. A method in accordance with claim 1, wherein thedirection of the beam is modified to produce prismatic structures in thedeveloped resist.
 16. A method in accordance with claim 1, wherein theexposure comprises a multiplicity of exposure steps with ions of adifferent energy or a different type.
 17. A method in accordance withclaim 1, wherein a sequence of alternating exposure and developmentsteps are applied a multiplicity of times to the same resist material,to create resist voids with a three dimensional geometry.
 18. A methodin accordance with claim 1, wherein the exposed resist is not developedimmediately and wherein an additional layer of resist is made to adhereto the original resist surface following which a further exposure ismade after which all regions of the exposed resist which are accessibleto the developer are developed.
 19. A method in accordance with claim 1,wherein a multiplicity of resist layer additions and associatedexposures are performed, following which all regions of the exposedresist which are accessible to the chemical developer are developed. 20.A method in accordance with claim 1, wherein a metallic or metallisedsubstrate material underlies the resist layer or layers, and followingdevelopment, the voids within the resist are filled, either partially orwholly, by electroplating metal onto the exposed areas of the underlyingmetal substrate to produce metal microstructures.
 21. A method inaccordance with claim 20, wherein the metallised substrate comprises asemiconducting material, in which electronic devices have beenfabricated to produce micromechanical devices with integratedelectronics.
 22. A method in accordance with claim 20, wherein theproduced metal microstructures are used to create microstructures inother materials by the process of micromoulding.
 23. A method inaccordance with claim 1, wherein one or more sacrificial layers areemployed such that, after removal of the sacrificial layer or layers,metal microstructures are formed which are either partially or whollydetached from an underlying metal substrate.
 24. A method in accordancewith claim 1, modified by the use of a nagative resist material which isexposed, thereby rendering the exposed the resist material inert to achemical developer which is highly specific in the removal of theunexposed material while leaving the exposed material substantiallyunaffected.
 25. A method in accordance with claim 1 wherein the resisthas been structured by a prior micromachining process.
 26. A method inaccordance with claim 1 wherein an exposure pattern is definedlithographically using a proximity mask with open areas through whichthe incident ions in the beam pass unaffected thereby exposing theunderlying resist, and other areas composed of material of sufficientthickness and density to completely absorb the incident ions therebyleaving the resist underlying such regions unexposed.
 27. A method inaccordance with claim 26, modified by using a patterned proximity maskwhich has some areas composed of material of sufficient thickness toallow transmission of the ions but at reduced energy.
 28. A method inaccordance with claim 27, using a beam of light ions having an energygreater than 250 KeV to expose a resist material as a step inmicromachining structures.
 29. A method in accordance with claim 1,wherein an exposure pattern is defined by a lithographic processemploying projection, by means of an image forming system, of the imageof a mask which selectively absorbs or scatters ions, onto the surfaceof the resist.
 30. A method in accordance with claim 1, wherein thecomponent is micromechanical, micro-optical, micro-fluidic,micro-electric, micro-acoustical or micro-chemical.
 31. Amicromechanical component produced by the method of claim
 1. 32. Amethod for preparing a component comprising exposing a defined area of amaskless resist material to produce a chemical change in a property ofsaid area enabling said area to be selectively removed or rendered inertwherein the defined area is exposed with a high energy beam of lightions, the beam having an energy greater than 250 KeV, wherein the lightions comprise isotopes of hydrogen, helium or lithium.
 33. A method inaccordance with claim 32, wherein the beam of light ions is parallelwith a diameter within the range of about 100 nanometers to 10micrometers.
 34. A method for preparing a component comprising exposinga defined area of a maskless resist material to produce a chemicalchange in a property of said area enabling said area to be selectivelyremoved or rendered inert wherein the defined area is exposed with ahigh energy beam of light ions, the beam having an energy greater than250 KeV, wherein the ions are electrically or magnetically focused toproduce a parallel beam, which is scanned over the surface of themaskless resist material in a predefined pattern thereby producing threedimensional microstructures in the developed material resist, whereinthe microstructures produced in the developed resist material have ahigh aspect-ratio.
 35. A method in accordance with claim 34, wherein theparallel beam has a diameter within the range of about 100 nanometers to10 micrometers.